Place the sliced radishes and fennel in a bowl of ice water and let soak until they become crisp, about 15 minutes. Once they are crisp, drain and pat dry with paper towels, then toss them together in a medium bowl and set aside.

Place the onion and lemon juice in a small bowl and let soak 25-30 minutes, until the onion and the juice start to turn pink. Strain the onions over a blender so the blender catches the lemon juice. Add the onions to the bowl with the radishes and fennel.

With a sharp knife, remove the grapefruit and orange peel, including the bitter white pith. Working over a different bowl, cut the citrus segments free from the membranes and release the segments into the bowl. Squeeze the citrus juice from the membranes. Transfer the citrus into the bowl with the radish and fennel.

Add 3-4 tablespoons of the citrus juice, 1 cup radish leaves (thoroughly cleaned), and half of the avocado (use half if it is a large avocado, all of it if it is small) to the blender, and blend until smooth and creamy. Add more citrus juice to thin to your liking.

Gently fold enough of the dressing into the salad to coat the vegetables lightly and evenly. (Store the remaining dressing in an airtight container in the fridge and add to beans, grains, other veggies…) Add salt and pepper to taste, and garnish with the walnuts and small pieces of the fennel fronds.